The Prevalence of Cassava Mosaic Begomoviruses in Malawi

Cassava (Manihot esculenta) is an important staple crop for farmers in the tropics due to its drought tolerance, low inputs requirements and flexibility in planting, harvesting and storage. However, sustainable production of the crop is hampered by Cassava Mosaic Disease (CMD) which reduces tuber yield by up to 95%. CMD is caused by several species of circular single-stranded cassava mosaic begomoviruses (CMBs). Two comprehensive surveys which employed field leaf incidence evaluation and viral DNA detection methods were done in 2006/7 and 2010 in order to determine the distribution and diversity of CMBs in Malawi. The results revealed wide spread prevalence of CMD and CMBs throughout the cassava growing areas thus threatening cassava production. Although, CMD severity varied significantly among sampled districts (X2=11.37; P≤0.05), the mean severity (2006/7; 2.95, 2010; 3.9) for both surveys illustrated that the disease was severe with pronounced leaf mosaic and distortion patterns. Altitude and severity had a weak insignificant correlation suggesting that CMD is likely to be severe regardless of where cassava fields are in the country (2006/7 r= 0.05; P=0.87, 2010 r=0.12; P=0.70). Though significantly variable among districts (X2=9.87; p≤0.05), white flies population did not significantly affect percentage leaf incidence and CMD severity indicating that use of clean planting materials was critical for the management of the disease (leaf incidence r=0.14; P=0.67, severity r=0.17; P=0.61). DNA analysis showed that almost all CMD in the country is caused by EACMV-like viruses which represented 98.5% and 86% of all begomoviruses detected in 2006/7 and 2010 respectively. In view of the continued widespread occurrence of CMD-causing viruses in Malawi, greater emphasis needs to be placed on the development and application of control measures.